An exhaust gas filter to trap hazardous particulate matter (i.e. PM) contained in the exhaust gases is disposed in an exhaust pipe of an internal combustion engine, such as a vehicle engine. The exhaust gas filter includes a plurality of cells which have partition walls.
The cells may be designed to have a first and a second group thereof in an axial direction, which are in alignment in a honeycomb structure. In general, the cells comprise first and second cells. Some of the first cells are formed in such a way that upstream end faces of the first group, which face an exhaust gas flow, are respectively plugged by plug portions. Some of the second cells are formed in such a way that downstream end faces of the second group are respectively plugged by the plug portions. The partition walls are composed of porous material, and the exhaust gas flows into openings of the first group which faces the exhaust gas flow. The PM contained in the exhaust gases is removed by passing the exhaust gases via a partition wall which is disposed between the first and the second group. After this, the exhaust gases flow from the first group of the first cell via the partition walls and are discharged from the second group of the second cell.
In the exhaust gas filter of the above-mentioned structure, pressure loss when passing the exhaust gases through the filter easily increases in relation with an accumulation of the PM on the exhaust gas filter. Engine oil and trace impurities (e.g., S and Ca, or the like) contained in fuels generate ash. The ash carried with the exhaust gases into the exhaust gas filter also increases the pressure loss in accordance with the accumulation of the ash on the exhaust gas filter. Therefore, as shown in WO 2012/046486, for example, the exhaust gas filter has been proposed to be formed in a single-side plugging type structure. The single-side plugging type structure is formed in such a way that any some the upstream end faces of the cells in a honeycomb structure are respectively plugged by plugs. In the single-side plugging type structure, the accumulation of the ash may be restrained to discharge the ash from the first cells.
However, in the single-side plugging type structure of the exhaust gas filter, the exhaust gas flow which permeates the partition walls decreases in a case of a high exhaust gas flow velocity and the exhaust gases easily passes through an inside of the cells without exhaust gases being trapped. That is, the exhaust gases in the exhaust gas filter introduced from the upstream side to the downstream side of the exhaust gas flow without being passed through the partition walls. Therefore, appropriately selecting the length of the base material of the exhaust gas filter to restrain performance degradation of trapping the PM has been considered. The base material will be also referred to as a part of the partition walls. There will be a problem that the exhaust gas filter needs to increase in size.
Carrying catalyst on the exhaust gas filter while removing the PM contained in the exhaust gases at the same time can be considered. However, in the single-side plugging type structure, the exhaust gas flow which permeates the partition walls becomes smaller due to pores in the partition walls being coated by catalyst coating layers. There will be a problem that when the decrease of the exhaust gas flow is decreased, an amount of the catalyst coat is restricted. Therefore, desired purifying performance can not be obtained.